On the market today, multi-layer tubes are used for the inner shaft of a catheter. They include, for example, multi-layer polytetrafluoroethylene (PTFE) tubes, co-extruded HDPE (high density polyethylene) tubes and the like, which comprise a polyamide-based outer layer to permit these inner shafts to be welded to other tubes that have a second tube layer made of a polymeric, thermoplastic material.
Fluorinated polymers (e.g., PTFE) generally have the lowest coefficients of friction relative to other materials, are resistant to ageing and are highly resistant to chemicals. Disadvantages of PTFE are the complex manufacturing process of tubes (not by conventional extrusion since PTFE cannot be processed thermoplastically because PTFE is not fusible) and the low abrasion resistance. Another disadvantage of PTFE-based inner tubes is the high price and the dependence on certain manufacturers. A further disadvantage of PTFE as a material for medical devices is that PTFE cannot be sterilized using radiation sterilization methods.
Other thermally deformable (extrudable), fluorinated polymers that also have low coefficients of friction (for example, ETFE (polytetrafluorothylene+ethylene, E-CTFE (polychlorotrifluoroethylene+ethylene), PFA (polytetrafluoroethylene+perfluoropropylether), FEP (polytetrafluoroethylene+perfluoropropylene), PCTFE (polychlorotrifluoroethylene), PVF (polyvinyl fluoride) and PVDF (polyvinylidene fluoride)) cannot be welded to the established catheter polymers (such as polyamide 12 or PEBA, for example), which are used for outer shafts, tips and balloons, for instance, and are very difficult to bond due to the low surface tension and resulting reduced wettability.
For this reason, only highly complex, multi-layer tube structures have become established for use as inner shafts for RX catheters (with reduced friction with respect to guide wires), for example. Tubes are known, inter alia, which have PTFE as the inner layer and a polyamide-based outer layer or multi-layer structures having an HDPE inner layer and, likewise, a polyamide-based outer layer.
The disadvantage of co-extruded inner shaft designs based on HDPE, for example, is also the expensive manufacturing process by co-extrusion of at least two, typically three layers, in which one extruder is used for each layer.
Document US 2010/0063476 A1 describes the use of modified PVDF as the inner layer of co-extruded tubes. Co-extrusion requires the use of highly complex equipment and requires working with at least two, typically three separate extruders, which must be matched to one another, to ensure that the two or three co-extruded layers have the desired layer thickness distributions in the final tube dimensions. The temperatures of the separate extruders must be selected such that the different tube layers are connected to one another during extrusion, which, in the case of a 3-layer extrusion, typically takes place by way of a physical connection between the HDPE inner layer to the middle layer and by way of a chemical connection of the middle layer to the polyamide-based outer layer. The pressure and temperature requirements are therefore high. Co-extrusion results in covalent bonding of the two tube layers, which is absolutely desired.